Amazon often sells the same thing in a variety of editions — a book can be bound in hardcover or softcover or distributed purely electronically in Kindle format; a movie can be available on DVD or VHS or for download; all sorts of physical objects from MP3 players to backpacks come in a variety of colors — and since you’re ostensibly reviewing the essence of a product, not its format or color, they often combine reviews across edition/platform/color.
Sometimes this is useful; sometimes it’s harmful. In a book review, if I’m talking about how good the writing itself is, that applies to all editions of a book, but one format sometimes has specific flaws the others don’t.
(Especially for travel guidebooks you’ll see 1 or 2 star reviews from kindle users complaining about poor map quality. If you’re shopping for the paper copy; this won’t matter to you at all; if you’re shopping for the Kindle copy, this issue deserves to be brought to the forefront. In these cases the low reviews need to be glaringly obvious to kindle purchasers and not seen at all by paper purchasers; mixed together, they get the same visibility to everyone, which is to say, too little to the first group and too much to the second.)
For physical objects, for example camera backpacks, I’ve seen the problem more often in the opposite direction — an item offered in 3 colors might have 1 page with reviews and ordering information for all 3 colors, or might have 3 separate pages, fragmenting the reviews and leading to some really low sample sizes.
Update 2010/11/21: the Lonely Planet guidebook for Chile, which I bought for Kindle on 2010/10/21 and which prompted this post, is no longer available on amazon.com for Kindle, and the paper version how has at least 3 different editions for different years which are reviewed separately. I’m not sure why the Kindle one was taken down, but that’s an improvement in the treatment of the paper version. It looks like someone else noticed the problem the same time I did.